3d 1.2版本，sprite改用材质后，复制对应节点就报错材质为空，Cannot read property ‘_techIdx’ of null
[Scene] TypeError: Cannot read property ‘_techIdx’ of null
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at MaterialInstance.copy (d:\CocosCreator\CocosDashboard\resources.editors\Creator3D\1.2.0\resources\resources\3d\engine\bin.cache\dev\cocos\core\assets\material.js:423:29)
at new MaterialInstance (d:\CocosCreator\CocosDashboard\resources.editors\Creator3D\1.2.0\resources\resources\3d\engine\bin.cache\dev\cocos\core\renderer\core\material-instance.js:78:13)
at Sprite.getUIMaterialInstance (d:\CocosCreator\CocosDashboard\resources.editors\Creator3D\1.2.0\resources\resources\3d\engine\bin.cache\dev\cocos\core\components\ui-base\ui-renderable.js:143:33)
at UI.autoMergeBatches (d:\CocosCreator\CocosDashboard\resources.editors\Creator3D\1.2.0\resources\resources\3d\engine\bin.cache\dev\cocos\core\renderer\ui\ui.js:563:49)
at UI.commitComp (d:\CocosCreator\CocosDashboard\resources.editors\Creator3D\1.2.0\resources\resources\3d\engine\bin.cache\dev\cocos\core\renderer\ui\ui.js:437:16)
at Sprite._render (d:\CocosCreator\CocosDashboard\resources.editors\Creator3D\1.2.0\resources\resources\3d\engine\bin.cache\dev\cocos\ui\components\sprite.js:271:16)
at Sprite.updateAssembler (d:\CocosCreator\CocosDashboard\resources.editors\Creator3D\1.2.0\resources\resources\3d\engine\bin.cache\dev\cocos\core\components\ui-base\ui-renderable.js:262:16)
at UI._preprocess (d:\CocosCreator\CocosDashboard\resources.editors\Creator3D\1.2.0\resources\resources\3d\engine\bin.cache\dev\cocos\core\renderer\ui\ui.js:704:18)
at UI._walk (d:\CocosCreator\CocosDashboard\resources.editors\Creator3D\1.2.0\resources\resources\3d\engine\bin.cache\dev\cocos\core\renderer\ui\ui.js:658:14)
at UI._walk (d:\CocosCreator\CocosDashboard\resources.editors\Creator3D\1.2.0\resources\resources\3d\engine\bin.cache\dev\cocos\core\renderer\ui\ui.js:666:18)
